The invention resides in an exhaust gas turbocharger for an internal combustion engine with a turbine housing including an axially movable slide member with guide vanes movable into, and out of, the exhaust gas flow to the turbine wheel.
Such an exhaust gas turbocharger is disclosed in DE 43 32 400 C1, wherein the exhaust gas turbocharger turbine and the exhaust gas flow can be controlled by the guide vanes of the slide member depending on the operating state of the internal combustion engine. In a low rpm range of the internal combustion engine and during engine braking operation, the guide vane structure is for example inserted into the exhaust gas flow path such that the guide vanes form a radial flow guide structure in the annular exhaust gas admission flow path to the turbine wheel.
In this way, an increased charge air pressure is obtained in the associated compressor since the exhaust gas flow speed into the turbine is increased by the flow guide structure.
In the intermediate and in the upper speed range of the engine, the guide vanes are removed from the exhaust gas inlet passage to the turbine wheel whereby the flow cross-section thereof is increased. In this way, the charge air pressure in the compressor remains within the required limits preventing damage to the turbine components. The axial slide member with the guide vane structure provides actually for a turbine with a two stage operability.
Any exhaust gas turbocharger is to be adapted to the particular operating conditions of the associated internal combustion engine, particularly as far as flow rates and charge pressures are concerned. In order for the compressor to reach a desired charge pressure, the turbine must be designed with a particular excess size. Then however, it is disadvantageous that the efficiency of the relatively large turbine during braking operation of the internal combustion engine is too low. For this reason, it is known to provide various devices and measures which generate a high back pressure in the exhaust of the internal combustion engine whereby the braking power is increased.
Another disadvantage of excess size turbines is that, upon removing the guide vane structure from the radial annular gas admission flow channel, there is a relatively large jump to a lower pressure level. This means that a correspondingly long time is required for reaching a predetermined maximum charge pressure.
DE 32 44 928 discloses an exhaust gas turbocharger for an internal combustion engine wherein an air pressure release line extends between the charge air manifold and the exhaust gas manifold. Excess charge air under pressure can be discharged this way to the turbine exhaust side. In addition, a control valve arrangement is provided including a pressure release valve and a flow control valve.
DE 36 06 944 A1 discloses an exhaust gas turbocharger with a turbine housing which includes a radial gas inlet and an axial gas outlet channel. A bypass line interconnecting the gas inlet and gas outlet channels leads to an annular recess in the wall of the gas outlet channel. For controlling the bypass amount, an annular rotary valve is arranged in the recess and includes a radial control passage which can be brought, by rotation of the annular rotary valve by means of a linkage, into, or out of, alignment with the bypass line discharge opening.
For further general background information reference is made to DE 43 15 474 C1.
It is the object of the present invention to provide an exhaust gas turbocharger which will have a high efficiency during engine braking operation but which nevertheless generates an optimal charge pressure during operation of the internal combustion engine under full load.